Prosecutors say the men, three Germans and one Turkish national, were planning simultaneous attacks on discos and pubs at America's Ramstein air base in Germany, with about 10 times as much explosive as used in the 2005 attacks in London.

The suspected German-based Islamists will follow the proceedings from behind a glass shield in the high-security court in Dusseldorf, which has a helicopter landing platform on the roof to bring in the defendants.

Police say the men were well-advanced in their preparations for attacks and that more than 50 people could have been killed.

The CIA helped foil the plot after months of close co-operation with German authorities, according to reports.

The charges include preparing bomb attacks and being members of a terrorist organisation. If convicted, the defendants face up to 15 years in jail.

Defence lawyers will argue that information used in the investigation was obtained illegally through surveillance in Germany and Pakistan.

They will also say that prosecutors obtained testimony from other prisoners held in countries where inmates have been tortured.

Two of the defendants, Fritz Gelowicz, 29, and Daniel Schneider, 23, are German converts to Islam. Attila Selek, 24, is of Turkish origin and has German citizenship. The fourth suspect, Adem Yilmaz, has a Turkish passport.

The first witnesses, likely to be relatives of the defendants, are due to take the stand on Thursday. About 200 witnesses are due to give evidence during the trial which is expected to last up to two years.